With current observational constraints, the physical state of our
Universe, as understood in the context of the standard, or
Friedmann-Robertson-Lemaître-Walker (FLRW) model, can be crudely
extrapolated back to the Planck epoch
seconds after the Big Bang, beyond which the classical theory of
general relativity is invalid due to quantum corrections. At the
earliest times, the Universe was a plasma of relativistic
particles consisting of quarks, leptons, gauge bosons, and Higgs
bosons represented by scalar fields with interaction and symmetry
regulating potentials. It is believed that several spontaneous
symmetry breaking (SSB) phase transitions transpired in the early
Universe as it expanded and cooled, including: the grand
unification transition (GUT) at
seconds after the Big Bang (Here, the strong nuclear force split
off from the weak and electromagnetic forces. This also marks an
era of inflationary expansion and the origin of matter-antimatter
asymmetry through baryon, charge conjugation, and charge + parity
violating interactions and nonequilibrium effects.); the
electroweak (EW) SSB transition at
secs (when the weak nuclear force split from the electromagnetic
force); and the chiral (QCD) symmetry breaking transition at
secs during which quarks condensed into hadrons. The most stable
hadrons (baryons, or protons and neutrons comprised of three
quarks) survived the subsequent period of baryon-antibaryon
annihilations, which continued until the Universe cooled to the
point at which new baryon-antibaryon pairs could no longer be
produced. This resulted in a large number of photons and
relatively few surviving baryons. A period of primordial
nucleosynthesis followed from
to
secs during which light element abundances were synthesized to
form 24% helium with trace amounts of deuterium, tritium,
helium-3, and lithium.

By
secs, the matter density became equal to the radiation density,
identifying the start of the current matter-dominated era and the
beginning of structure formation. Later, at
secs (
years), the free ions and electrons combine to form atoms,
decoupling the matter from the radiation field as the Universe
continued to expand and cool. This decoupling or
post-recombination epoch marks the surface of last scattering and
the boundary of the observable (via photons) Universe. Assuming a
hierarchical (CDM-like) structure formation scenario, the
subsequent development of our Universe is characterized by the
growth of structures with increasing size. For example, the first
stars are likely to form at
years from molecular gas clouds when the Jeans mass of the
background baryonic fluid is approximately
, as indicated in Figure
1
. This epoch of pop III star generation is followed by the
formation of galaxies at
years and then galaxy clusters.

Figure 1:
Schematic depicting the general sequence of events in the
post-recombination Universe. The solid and dotted lines
potentially track the Jeans mass of the average baryonic gas
component from the recombination epoch at
to the current time. A residual ionization fraction of
following recombination allows for Compton interactions with
photons to
, during which the Jeans mass remains constant at
. The Jeans mass then decreases as the Universe expands
adiabatically until the first collapsed structures form
sufficient amounts of hydrogen molecules to trigger a cooling
instability and produce pop III stars at
. Star formation activity can then reheat the Universe and raise
the mean Jeans mass to above
. This reheating could affect the subsequent development of
structures such as galaxies and the observed Lyman-alpha clouds.